Polyethylene is produced commercially in a gas phase reaction in the absence of solvents by employing selected chromium and titanium-containing catalysts under specific operating conditions in a fluid bed process. The products of those original processes exhibited medium-to-broad molecular weight distribution. To be commercially useful in the gas phase fluid bed process, the catalyst must exhibit high activity, with concomittant high catalyst productivity, because gas phase process systems do not include catalyst residue removal procedures. Accordingly, catalyst residue in the polymer product must be so small that it can be left in the polymer without causing any undue problems in the fabrication and/or to the ultimate consumer. To this end, the patent literature is replete with developments of new catalysts, of high activity with corresponding high productivity values.
The use of metallocene compounds of transition metals as catalysts for polymerization and copolymerization of ethylene is one of those developments. Metallocenes can be described by the empirical formula Cp.sub.m MA.sub.n B.sub.p. These compounds in combination with methylalumoxane (MAO) have been used to produce olefin polymers and copolymers, such as ethylene and propylene homopolymers, ethylene-butene and ethylene-hexene copolymers, e.g., see Kaminsky et al, U.S. Pat. No. 4,542,199 and Sinn et al, U.S. Pat. No. 4,404,344, the entire contents of both of which are incorporated herein by reference. Unlike traditional titanium- and vanadium-based Ziegler-Natta catalysts, a metallocene, e.g. a zirconocene catalyst, free of titanium- and vanadium-components produce resins with very narrow molecular weight distributions (determined as MFR(I.sub.21 /I.sub.2) of 15 to 18, versus MFR of 25 to 30 for titanium-based catalysts) and with homogeneous short-chain branching distributions. When traditional titanium- and vanadium-based catalysts are used to copolymerize ethylene and higher alpha-olefins, the olefin is incorporated in polymer chains nonuniformly, and most of the olefin resides in the shortest polymer chains. With zirconocene catalyst, however, the branching distribution is essentially independent of chain length. LLDPE resins produced with zirconocene catalysts have superior properties. These resins can be used to make films with significantly better clarity and impact strength. Extractables of such resins are lower and the balance of properties in the films between the machine and transverse directions is excellent. More recently, as exemplified in U.S. Pat. No. 5,032,562, metallocene catalysts containing a second transition metal, such as titanium have been developed which produce bimodal molecular weight distribution products, having a high molecular weight component and a relatively lower molecular weight component. The development of a catalyst which can produce bimodal products in a single reactor is significant per se. That development also provides a commercial alternative to processes which require two reactors to produce bimodal production with production of one of the molecular weight components in a first reactor and transfer of that component to a second reactor and completion of the polymerization with production of the other component of different molecular weight.
Methylalumoxane (MAO) is used as co-catalyst with metallocene catalysts. The class of alumoxanes comprises oligomeric linear and/or cyclic alkylalumoxanes represented by the formula: R--(Al(R)--O).sub.n --AlR.sub.2 for oligomeric, linear alumoxanes and (--Al(R)--O--).sub.m for oligomeric cyclic alumoxane wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-20 and R is a C.sub.1 -C.sub.8 alkyl group and preferably methyl. Methylalumoxane is commonly produced by reacting trimethylaluminum with water or with hydrated inorganic salts, such as CuSO.sub.4 5H.sub.2 O or Al.sub.2 (SO.sub.4).sub.3.5H.sub.2 O. Methylalumoxane can be also generated in situ in polymerization reactors by adding to them trimethylaluminum and water or water-containing inorganic salts. MAO is a mixture of oligomers with a very wide distribution of molecular weights and usually with an average molecular weight of about 1200. MAO is typically kept in solution in toluene. While the MAO solutions remain liquid at fluid bed reactor temperatures, the MAO itself is a solid at room temperature.
Most of the experiments reported in the literature relating to methylalumoxane used as a cocatalyst with metallocene catalysts are undertaken in a slurry or solution process, rather than in a gas phase fluid bed reactor process.
It was found that the metallocene catalyst must contact the MAO cocatalyst while MAO is in solution in order for the catalyst to be activated in the fluid bed reactor. Moreover, it was discovered that extensive reactor fouling results when MAO solutions are fed directly into the gas phase reactor in large enough quantities to provide this liquid contact. The fouling occurs because the MAO solution forms a liquid film on the interior walls of the reactor. The catalyst is activated when it comes into contact with this liquid film, and the activated catalyst reacts with ethylene to form a polymer coating which grows larger in size until the reactor is fouled. In addition, since substantially all of the activation takes place on the walls, the MAO is not uniformly distributed to the catalyst particles. The resulting non-homogeneous polymerization gives low catalyst activity and poor product properties.